The present invention relates to a processing arrangement for a PAL digital video signal, and more particularly to an arrangement for processing a digital color subcarrier carrying chrominance signals such as U and V signals into which the color subcarrier is sampled on a discrete time base.
A PAL composite video signal contains a luminance (Y) signal, a (B-Y) color difference (U) signal and a (R-Y) color difference (V) signal. The U and V signals are simultaneously carried by a color subcarrier which is joined in the Y signal.
In general, a color video signal containing the Y signal and the color subcarrier is sampled at a rate, which is, for example, three, four or more times as much as a subcarrier frequency, to produce a digital color video signal. The digital color video signal is often used for various processing. With a digital video special effect system, for example, a video image is compressed and magnified under such digital processing. Digitization of the television signal may facilitate the video signal to be stored, read out and delayed with stable and desired characteristics. Frequently a selected sampling rate for digitization of the video signal is four times as much as the color subcarrier frequency. In the case of the sampling rate equal to four times the subcarrier frequency, a sampling axis is preferably set to .+-.I and .+-.Q axes on a color vector plane for the NTSC video signal, and to .+-.U and .+-.V axes for the PAL video signal.
In a PAL digital video signal, especially, in a digitized PAL color subcarrier, the sequence of samples is associated with the V, U, -V, -U axes and so on while the PAL video signal has not yet been encoded for broadcasting.
In the modulation of the PAL signal for broadcasting, the phase of the V signal, i.e. the (R-Y) color difference signal, is inverted from one to another horizontal line. Therefore, when the sequence of samples on one line is aligned by V, U, -V, -U axes and so on, the sequence of samples on the succeeding line is aligned by -V, U, V, -U axes and so on.
In the PAL system, furthermore, the relationship ##EQU1## is established between the subcarrier frequency f.sub.SC and the horizontal synchronous frequency f.sub.H so that each line includes 283.7516 subcarriers. When such the PAL color video signal is sampled at the rate equal to four times the subcarrier frequency, each line contains 1135.0064 (=283.7516.times.4) samples. Since the decimal fraction is so small as to be negligible, it may thus be considered that each line ultimately contains 1,135 samples or numbers of digital data, which number is an odd number. Consequently, the sequence of samples on one line starts with V, U, -V, -U axes and so on, and ends with the series of V, U and -V axes. That is, it ends with the -V axis so that a sampling point associated with the next -U axis will be involved in the successive line. In this manner, one sample is shifted from one line to the succeeding line. In addition, with the PAL system, modulation axes for V signals are inverted between successive lines as described above, resulting in losing correspondence in arrangement of samples between two successive lines.
The above-mentioned characteristics of the PAL system prevent direct arithmetic operations on digitally sampled data, such as addition, subtraction, averaging, etc., between successive lines. Rather than direct operations, a color decoder was conventionally used to restore original V and U signals, which are in turn operated on in various kinds of processing. Such provision for a color decoder results in complicating the circuit configuration and deteriorating the video information.